A typical vehicle fuel vapor recovery system found in a vehicle with a carbureted engine includes a vapor storage canister in which vapors from the fuel tank, and often from the carburetor float bowl as well, are adsorbed and stored, rather than being released to the atmosphere. These vapors are later purged from the canister by engine manifold vacuum and fed into a port in the throttle body located downstream of the throttle and burned in the engine. The withdrawal and burning of stored fuel vapors is generally controlled so as to in turn limit and control the richness of the fuel air mixture. For example, it would overly enrich the mixture if vapors were to be purged during engine idling. Therefore, it is desirable that purging begin only when, or at least be greatest when, the vehicle has reached a sufficient speed. Accordingly, the control system shown in U.S. Pat. No. 4,527,532 uses a speed sensor and a solenoid valve to increase the purging rate when that sufficient speed has been reached. While such an approach is obvious and is directly tailored to vehicle speed, it is expensive and somewhat complex, due to the electronics involved.
The more common approach is to control air fuel ratio with a strictly mechanical purge valve that is closed during engine idling, but which opens when the throttle opens, even though it takes some time after throttle opening for the vehicle to get up to speed. A typical example of a fuel vapor recovery system with such a control is illustrated in FIG. 7. A fuel tank 10 continuously feeds excess vapors to a storage canister 12. A throttle body 14 supports a carburetor bowl 16, and houses a throttle 18, with a manifold vacuum port 20 located downstream from throttle 18 and with a control vacuum port 22 located upstream from throttle 18. When throttle 18 is closed, as shown, the control vacuum port 22 is exposed to atmospheric pressure, but is exposed to manifold vacuum when throttle 18 is opened. A canister control valve, designated generally at 24, contains a purge valve, and also controls the vapor venting from carburetor bowl 16. Canister control valve 24 has a generally hollow body, and is ported to four lines, a canister line 26 that runs to canister 12, a carburetor bowl line 28 that runs to carburetor bowl 16, a manifold vacuum line 30 that runs to manifold vacuum port 20, and a control vacuum line 32 that runs to control vacuum port 22. Two internal spring and diaphragm valves, a vapor vent valve 34 and a purge valve 36, operate as follows. When the engine is off, there is no vacuum through line 30 or line 32, and both valves 34 and 36 are in the down position shown, meaning that vapor vent valve 34 is open, while purge valve 36 is closed. This allows fuel vapors to vent from carburetor bowl 16, through line 28 to line 26 and ultimately to canister 12, but blocks vapors from flowing from canister 12, through line 26 to line 30. When the engine has been started, but is only idling, throttle 18 will still be in the closed position shown, but there will be enough manifold vacuum through line 30 to close valve 34, and block vapors from venting from bowl 16. However, line 32 will still not be exposed to manifold vacuum, so purge valve 36 will remain closed, and there will be no vapor purging from canister 12. When throttle 18 opens, control vacuum port 22 becomes exposed to manifold vacuum, pulling up and opening purge valve 36. This allows vapors to purge from canister 12, through lines 26 and 30 and into throttle body 14 to be burned. As soon as throttle 18 recloses, port 22 becomes exposed to atmospheric pressure again, and purge valve 36 closes almost immediately, stopping the purging from canister 12. An optional thermal switch 38 in control vacuum line 32 prevents purging at all when the engine is cold.
The rapid closing of purge valve 36 upon the closing of throttle 18 is needed in order to prevent the fuel air mixture from becoming too rich during deceleration. However, the other side of the coin, the rapid opening of purge valve 36 upon the reopening of throttle 18, can cause a temporary over richness of the mixture. This is because, as noted, it takes some time after throttle opening for the vehicle and engine to get up to speed, and for the carburetor to develop sufficient airflow to be able to easily handle the increased fuel vapors from the canister. A partial solution is to put a one way air flow delay valve into the control vacuum line 32, between switch 38 and purge valve 36. Then, there will be a time delay between the opening of throttle 18 and the opening of purge valve 36, which will give the engine time to speed up before vapor purging begins. A shortcoming of this approach, however, is that in order to get sufficient delay, a fairly restrictive delay valve must be used. The purge valve 36 closes almost immediately when the throttle 18 closes, but that closing may be very short, with the vehicle and engine staying at substantially at the same speed, and thus more than capable of burning purged fuel vapors upon the reopening of throttle 18. However, the same, relatively long time delay in the reopening of purge valve 36 will occur as when it was initially opened, as the system has no way of distinguishing between the initial throttle opening and the reopening after only a short throttle closing. Delaying the reopening of purge valve 36 under these conditions, that is, when the engine is still more than capable of burning purged vapors, would undesirably reduce the degree of purging.